Abstract: An apparatus and method for treating exhaust gases. In this apparatus, a plurality of stages of reactor chambers (R.sub.1, R.sub.2, . . . . and R.sub.n) are connected in series in the direction of an exhaust gas flow. Further, high-voltage power supplies (V.sub.1, V.sub.21 . . . and V.sub.n) are connected to the reactor chambers (R.sub.1, R.sub.2, . . . . and R.sub.n), respectively. Moreover, in each of these reactor chambers, a streamer discharger plasma is generated. Furthermore, the more downstream a reactor chamber of a stage is placed, the lower energy to be cast into the reactor chamber becomes. The density of electrons generated in a gas decomposition unit is high in a portion thereof on the upstream side of the exhaust gas flow and the electron density is low in a portion thereof on the downstream side. Additionally, the present invention further provides a pulse generator in which a high voltage, which is an output voltage of a D.C. charger (V.sub.